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Tree Bark: That Beneficial Barrier


Tree bark is roughly akin to the skin of animals; it helps to define the boundary between the internal and external world, and it provides protection against a variety of threats. While bark fails to attract the same level of attention that roots, leaves, fruit and flowers do, it remains a critical component of tree biology, equally deserving of discussion.   

Anatomy of Bark

Tree bark contains two different cell layers. The innermost layer, called the phloem, transports sugars from sites of photosynthesis (the leaves) to the roots. The other primary layer of the bark is called the periderm. The periderm is itself composed of two distinct layers: The cork cambium, a site of rapid cell division, produces the phloem and the outer layer of dead cells, called cork

Bark Variation

A brief walk in the woods quickly reveals that barks differ greatly from one to the next. Birch (Betula spp.) bark is thin, paper-like and peels away from the trunk easily, while sequoia (Sequoiadendron giganteum) bark may be as much as 3 feet thick. While scientists suspect that some barks serve no particular adaptive function, it is likely that the bark of most species has evolved to provide protection from specific threats in the habitat. 

For example, trees from fire-prone regions often produce thick bark – giant sequoias provide an excellent example of this, as do many pines of the southeastern United States, who must also cope with periodic fires. Some botanists believe that trees bearing smooth bark often do so to prevent fungal colonization, and they cite the fact that many such species evolved in rainforest habitats, where these fungi are ubiquitous. However, other botanists believe the slippery bark serves to prevent insects from clinging to the bark and feeding on the tree’s delicate tissues. In some cases, such as manzanita trees (Arctostaphylos spp.), the thin, peeling bark also contains tannins, which help to dissuade insects. 

Benefiting Others

The benefits of bark are not limited to trees; many other creatures benefit from the protective covering as well. Squirrels, for example, often strip the bark of trees. Although the reasons they engage in this behavior are not yet fully understood, it is likely related to their need for additional nutrients. Of course, porcupines and beavers are famous for their bark-eating habits, but deer, rabbits and rodents also dine on bark when times are tough. 

Humans also benefit from tree bark. Although manufacturers currently synthesize aspirin in a lab, the active ingredient lies within willow bark, which was used to make analgesic teas for centuries. Currently, scientists are researching the chemical properties of pine tree bark, which contains compounds that may be helpful for treating melanoma. Some people, eager to find a cure for conditions that are hard to treat, have begun experimenting with tree bark extracts. The inner bark of some trees even serves as an emergency food source

Damaged Bark

Trees sustain damage to their bark from a variety of sources, including wildlife, pests, pathogens, lightning, hail, vandals, landscaping crews and construction activities. Bark damage exposes trees to myriad threats, and, if left untreated, the tree can ultimately die. However, with proper wound treatment and supportive care, trees are often able to produce callus wood at the site of the injury, which will eventually provide the tree with some protection. 

If one of your trees suffers damage to its bark, contact San Francisco's leading tree care service without delay. One of our ISA-certified arborists will inspect the damage, devise a treatment plan and help restore your tree's health. You must act fast though, as bark damage allows the delicate inner wood of a tree to dry out and every minute your tree goes without its protective bark barrier, it remains exposed to pests and pathogens. 



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